Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 October 2018 doi:10.20944/preprints201810.0178.v1 1 Type of the Paper: Review 2 Role of wetland plants and use of ornamental 3 flowering plants in constructed wetlands for 4 wastewater treatment: a review 5 6 Luis Carlos Sandoval-Herazo1-2, Josè Luis Marín-Muñiz3*, María Graciela Hernández y Orduñas1, 7 & Antonio Janoary Aleman-Chang1 8 1 División de Estudios de Posgrados e Investigación, Tecnológico Nacional de México/Instituto Tecnológico 9 de Orizaba, Oriente 9, Emiliano Zapata Sur, C.P. 94320 Orizaba, Veracruz, México;[email protected] 10 (L.C.S.-H.). 11 2 División de Estudios de Posgrados e Investigación, Tecnológico Nacional de México/Instituto Tecnológico 12 Superior de Xalapa, Sección 5A Reserva Territorial S/N, Santa Bárbara, 91096 Xalapa Enríquez, Veracruz, 13 México; [email protected] (L.C.S-H.); [email protected] (M.G.H-O.); 14 [email protected] (A.J.A-C.). 15 2 Department of Sustainability and Regional Development, El Colegio de Veracruz, Xalapa, Veracruz, Mexico 16 (J.L.M-M). 17 18 * Correspondence: [email protected]; Tel.: +52-228-162-4680 19 20 Abstract: The vegetation in constructed wetlands (CWs) plays an important role in wastewater 21 treatment. Popularly, the common emergent plants in CWs have been vegetation of natural 22 wetlands. However, there are ornamental flowering plants that have some physiological 23 characteristics similar to the plants of natural wetlands that can stimulate the removal of pollutants 24 in wastewater treatments; such importance in CWs is described here. A literature survey of 87 CWs 25 from 21 countries showed that the four most commonly used flowering ornamental vegetation 26 genera were Canna, Iris, Heliconia and Zantedeschia. In terms of geographical location, Canna spp. is 27 commonly found in Asia, Zantedeschia spp. is frequent in Mexico (a country in North America), Iris 28 is most commonly used in Asia, Europe and North America, and species of the Heliconia genus are 29 commonly used in Asia and parts of the Americas (Mexico, Central and South America). This 30 review also compares the use of ornamental plants versus natural wetland plants and systems 31 without plants for removing pollutants (COD, BOD, nitrogen and phosphorous compounds). The 32 removal efficiency was similar between flowering ornamental and natural wetland plants. 33 However, pollutant removal was better when using ornamental plants than in unplanted CWs. The 34 use of ornamental flowering plants in CWs is an excellent option, and efforts should be made to 35 increase the adoption of these system types and use them in domiciliary, rural and urban areas. 36 Keywords: Ornamental flowering plants, constructed wetlands, wastewater, pollutants. 37 38 1. Introduction 39 Nowadays, the use of constructed wetlands (CWs) for wastewater treatment is an option 40 widely recognized. This sustainable ecotechnology is based on natural wetland processes for the 41 removal of contaminants, including physical, chemical and biological routes, but in a more 42 controlled environment compared with natural ecosystems [1,2,3]. These ecologically engineered 43 systems involve three important components: porous-filter media, microorganism and vegetation 44 [2]. The mechanisms for the transformation of nutrient and organic matter compounds are © 2018 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 October 2018 doi:10.20944/preprints201810.0178.v1 2 of 20 45 conducted for biofilms of microorganisms formed in the porous media and the rhizosphere zone 46 [4,5]. The media materials (soil, sand, rocks, and gravel) provide a huge surface area for 47 microorganisms to attach, contributing to macrophyte growth, and also act as filtration and/or 48 adsorption medium for contaminants present in the water [6]. In regards to the vegetation, one of the 49 most conspicuous features of wetlands is the role that plants play in the production of underground 50 organisms (i.e. rot and rhizomes) in order to provide substrate for attached bacteria and oxygenation 51 of areas adjacent to the root, and absorb and adsorb pollutants from water. Nitrogen (N), 52 phosphorus (P) and other impurities are mainly taken up by wetland plants through the epidermis 53 and vascular bundles of the roots, and are further transported upward to the stem and leaves [7]. 54 This provides carbon for denitrification during biomass decomposition and prevents pollutants 55 from being released from sediments [8,9,10]. The use of the CWs technology began in Europe during 56 the 1960’s. [1], and has been replicated on other continents. The type of vegetation used are plants 57 from natural wetlands, inlcuding Cyperus papyrus, Phragmites australis, Typha and Scirpus spp., which 58 have been evaluated for their positive effects on treatment efficiency for nutrient and organic 59 compounds around the globe [8,9,11]. In the Americas, such species are typical in CWs, and are 60 found mainly in the United States, where the technology has been used extensively and is 61 implemented in different rural and urban zones [12,13,14,15,16]. In recent studies (15 years ago), the 62 goal of CWs studies involved an investigation into the use of herbaceous perennial ornamental 63 plants in CWs, including the use of species with different colored flowers to make the systems more 64 esthetic, and therefore, making it more probable for adoption and replication. 65 66 This review attempts to study the role of macrophytes in CWs and highlights the use of 67 ornamental flowering plants in this type of ecotechnology around the world. This includes plants 68 that are not typical in natural wetlands, and shows the resulting removal efficiency and their 69 importance in rural communities. The aim of this study is to create a context regarding the 70 advantages that the use of CWs with ornamental flowering plants provides, emphasizing that these 71 systems could be used for more sites that require wastewater treatment. The information from 87 72 constructed wetlands using ornamental flowering plants (OFP) in 21 countries was reported in the 73 literature that was analyzed. Only published or accepted (in press) papers were considered; the 74 results of theses or abstracts of conferences were not considered. 75 2. Role of macrophytes in CWs 76 The plants that grow in constructed wetlands have several properties related to the water 77 treatment process that make them an essential component of the design. Macrophytes are the main 78 source of oxygen in CWs through a process that occurs in the root zone, called radial oxygen loss 79 (ROL) [17]. The ROL contributes to the removal of pollutants because it favors an aerobic 80 micro-environment, and waste removal is therefore accelerated, whereas in anaerobic conditions 81 (the main environment in CWs) there is less pollutant removal. In a recent study [18] comparing the 82 use of plants in high density (32 plants m-2) and low density (16 plants m-2) CWs, the removal of 83 nitrogen compounds in high density CWs was twice that of CWs using a low density of plants, 84 which is strong evidence of the importance of plants in such systems. The removal rate of total 85 nitrogen (TN) and total phosphorous (TP) were also positively correlated with the ROL of wetland 86 plants, according to a study involving 35 different species [19]. 87 88 The roots of plants are the site of many microorganisms because they provide a source of 89 microbial attachment [8] and release exudates, an excretion of carbon that contributes to the 90 denitrification process. This is exudates a necessary source of carbon, which increases the removal of 91 pollutants in anoxic conditions [20,21]. Other physical effects of root structure on CWs includes a 92 reduction in the velocity of water flow, promotion of sedimentation, decreased resuspension, 93 prevention of medium clogging and improved hydraulic conductivity [5,2]. A 5 year study 94 evaluated the influence of vegetation on sedimentation and resuspension of soil particles in small Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 October 2018 doi:10.20944/preprints201810.0178.v1 3 of 20 95 CWs [22]. The author showed that macrophytes stimulated sediment retention by mitigating the 96 resuspension of the CWs’ sediment (14 to 121 kg m-2). Macrophytes increased the hydraulic 97 efficiency by reducing short-circuit or preferential flow. Plant presence led to decrease saturated 98 hydraulic conductivity in horizontal subsurface flow. This study was imperative, since monitoring 99 macrophytes is essential for understanding and controlling clogging in CWs [23]. 100 101 The removal of organic and inorganic pollutants in CWs is not only the role of microorganisms. 102 This function is also exerted by plants, which are able to tolerate high concentrations of nutrients 103 and heavy metals, and in some cases, plants are able to accumulate them in their tissues [24]. It has 104 been estimated that between 15 and 32 mg g-1 of TN and 2–6 mg g-1 (dry mass) of TP are removed by 105 CW plants, which was measured in the aboveground biomass[25,26] . 106 Other uptakes of xenobiotic compounds (organic pollutants) are also the result of the presence 107 of plants, involving processes such as transformation, conjugation and compartmentation [24]. 108 109 3. Survey results of use of ornamental flowering plants in CWs 110 111 Table 1 lists examples of ornamental plants used in CWs around the world that were designed 112 for the removal of various types of wastewater. OFP have been used in some countries, particularly 113 in Mexico and China. In China, the most popular vegetation used is Canna sp., while in Mexico the 114 ornamental plant used is more diverse, including plants with flowers in different colors, shapes and 115 aromatic characteristics (Canna, Heliconia, Zantedeschia, Strelitzia spp).